The effect of CO conversion (35-91 %) on hydrocarbon product distribution (CH4, C-5(+) and olefin selectivities) and CO2 selectivity was studied in a fixed bed reactor on 15 % Co/Al2O3 catalyst at conditions of industrial relevance. Selectivity to CH4 decreased, whereas those of C-5(+) and CO2 increased with increasing CO conversion (and consequently, partial pressure of water produced during the Fischer-Tropsch synthesis). 1-Olefin content of C-2-C-5 olefins decreased and 2-olefin content of C-4 hydrocarbon increased with increasing CO conversion.